DUTCH ELM DISEASE,
Importance.-Dutch elm disease primarily affects American and European species of elm, and is a major disease problem throughout most of the range of elm in the United States. The greatest economic loss results from death of high value urban trees.
Identifying the Fungus.-No fruiting bodies of this fungus are seen in the field. In the laboratory the fungus readily produces easily identified, spore bearing structures.
Identifying the Injury.-Symptoms of the disease include wilting, yellowing, and browning of the leaves, brown or purplish brown streaking of the wood under the bark, and crown dieback. Symptoms normally progress rapidly through the crown. Complete wilting often occurs within six weeks of infection.
Biology. -The fungus is transmitted to healthy trees in two ways: bark beetles transmit spores from diseased to healthy trees, or the fungus grows through root grafts between diseased and healthy trees. Generally, death of the infected tree is rapid. However, some asymptomatic trees have been found that had been infected for several growing seasons. The specific manner in which the fungus kills trees is unknown. The vascular system of the infected tree is affected, reducing the conduction of water and nutrients.
Control. -The most available control is removing infected trees and promptly destroying the wood. If infected wood is to be used as firewood, it should first be debarked. Trenching to disrupt root grafts is recommended to protect healthy elm trees near diseased ones.
In urban situations, insecticide spraying of high value trees has been effective in keeping bark beetles from attacking susceptible trees. Space trees further apart to prevent root grafts or use mixed tree species in ornamental plantings.
Sterilize pruning equipment before use from one elm to the next to prevent spreading the fungus.